iio: st_sensors: use the helper function

iio: st_sensors: use the helper function

The ST sensors can be used as a trigger for its own triggered buffer
but it is also possible to use an external trigger: a HRTimer or
even a different sensor (!) as trigger. In that case we should not
pick the timestamp from our own interrupt top half even if it is
active.

This could practically happen if some other sensor is using the
ST sensor as trigger but the ST sensor itself is using e.g.
an HRTimer as trigger. So the trigger is on, but not used by us.

We used to assume that whenever the hardware interrupt is turned
on, we are using it for our own trigger, but this is an
oversimplification.

Handle this logically by using the iio_trigger_using_own() helper.

Cc: Giuseppe Barba <giuseppe.barba@st.com>
Cc: Denis Ciocca <denis.ciocca@st.com>
Cc: Crestez Dan Leonard <leonard.crestez@intel.com>
Cc: Gregor Boirie <gregor.boirie@parrot.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>